It's difficult for me to write a review of Reply 1988 because it's so close to my heart and has been for so long that I've lost any objective distance from which to review it. Even if I love a show I can usually see where it had flaws or weaknesses and rate it accordingly, but with this show I just need to give it that perfect 10/10 rating. It would be sacrilege not to.
This was the first (and one of the only) truly great dramas I have ever seen. It is also one of the few dramas I have recommended/shown to people in my life who do not watch asian dramas (like my mom). This is such a great showcase for the strength of a typical k-drama format: 20 long episodes, released consecutively, makes the show feel lengthy and significant (because it is, at almost 30 hours? of runtime) while still being self-contained. The themes can also stay very pure, entering around family and growing up in the most emotionally fulfilling possible way.
The cast is very good. Both the kids and the parents are so enjoyable to watch, both on their own and in those large and complicated group dynamics. There are no scenes to skip, no glaringly weak acting that brings down the ensemble. Everyone ranges from solid to excellent and there are more than ten very memorable and well-acted scenes I can think of off the top of my head. This great cast is supported by great writing and directing. There are so many understated, subtle moments that have a huge emotional impact, they don't need to rely on unrealistically high stakes for there to be these incredible dramas in the show. The show doesn't manufacture comas and car accidents and divorces and long-lost twins because they don't need to. They're bringing out the drama and emotion of daily life, and all the trials and joys of regular people and their relationships.
I would say my only quibble is that I'm not very fond of the focus on who Deok-sun ends up marrying. The Reply series always has some variant of this question built in to the fabric of the show, be it Reply 1997, 1994 or 1988, but I think it feels most glaringly out of place here. It seems cheap and unnecessary and if I could edit out the 'future' interview-style scenes I would. The show stands perfectly well without them. That said, they are not terrible or anything, I just think it puts undue focus on a love triangle that would've probably felt more organic otherwise.
Overall, this is the show that I recommend to people who don't watch dramas. Period. Everyone I have shown this to has enjoyed it, and there is a reason that it always ends up on those 'greatest dramas of all time' lists. It may take an episode or two to get used to the pace and the setting and characters, but once you do you'll be a fan for life.